The present invention relates to a method for controlling a variable displacement pump applied to a machine, such as a construction machine using a bleed-off hydraulic system.
For a hydraulic circuit used in the field of construction machines including a hydraulic excavator, the applicant has proposed in JP-A-2007-205464 a method for controlling a variable displacement pump, driven by an engine, the pump discharge flow rate of which can be adjusted from the outside, having actuators connected thereto through a plurality of closed-center-type directional control valves, respectively, the closed-center-type directional control valves controlling the variable displacement pump by electrical calculation in place of center-bypass-type directional control valves.
This method intends to mathematically replace a bleed-off characteristics part of a conventional bleed-off hydraulic system including center-bypass-type directional control valves, which is a part for controlling pressure and flow rate for actuators, by controlling discharge pressure of the variable displacement pump by calculation by controller. A conventional variable displacement pump is controlled with some of hydraulic fluid pumped by the variable displacement pump, being actually returned to a tank, failing to effectively utilize the variable displacement pump. However, controlling discharge pressure of a variable displacement pump by calculation by controller as if the variable displacement pump has a bleed-off characteristic allows exclusion of center-bypass paths from directional control valves and discharge of hydraulic fluid with only an actually required flow rate.
For the method for controlling a variable displacement pump described in JP-A-2007-205464, an example is described in which, when calculating a pump discharge pressure specified value (virtual commanded pump discharge pressure Pidea), a pump discharge flow rate Qidea is limited by an upper limit that is an engine horse power He divided by the commanded pump discharge pressure Pidea, as shown in FIG. 7. However, as seen from characteristic curve defining the relation between pump discharge pressure P and discharge flow rate Q (hereinafter sometimes simply referred to as “characteristic curve”) shown in FIG. 8, the variable displacement pump is usually controlled such that the pump discharge flow rate Q is kept constant when the pressure P is less than or equal to a predetermined pressure P1, and the product of the discharge pressure P and the discharge flow rate Q is kept constant when the pressure P exceeds the pressure P1. Due to this, when the flow rate of hydraulic fluid supplied to an actuator is low and the discharge pressure P has exceeded the pressure P1, even though the pump discharge flow rate Q can be increased, the obtained calculation result suggests decreasing the commanded pump discharge pressure Pidea, limiting the pump discharge flow rate Q, which may prevent effective utilization of the variable displacement pump.
Also, for the method for controlling a variable displacement pump described in JP-A-2007-205464, another example is described in which the pump discharge pressure specified value (commanded pump discharge pressure Pidea) is calculated without taking the engine horse power into consideration, as shown in FIGS. 9(a) and 9(b). However, absolutely without taking the engine horse power into consideration, a load of the variable displacement pump may become too high, leading to engine stall.